When you have irregular cycles with a large range, it is ideal to stick to the ultra sonogram dates. Your last period appeared after a gap of 40 days and your date of ovulation is difficult to calculate owing to the persistent disparity in the length of the cycles. It would have been possible if you were recording your basal body temperature
in your last cycle before conception. So, the only choice left is to go by the sonogram dates and this is ideal if done at the earliest calculation. Good luck.